gistfile1.rb

class User < ActiveRecord::Base
  belongs_to :userable, :polymorphic => true
end

class Designer < ActiveRecord::Base
  has_one :user_login, :as => :userable
  accepts_nested_attributes_for :user_login
end

class DesignersController < ApplicationController
  def new
    @designer = Designer.new
    #@designer.user_login.build
    respond_to do |format|
      format.html
    end
  end
end

# View uses f.fields_for :user_login do |login|